40 Embryonic development and Larvai Stages. 



After the 4-cell stage the cleavage does not affect the yolk, and the cells at the ani- 

 mal pole go on rapidly dividing in ali directions so as to forni a continuous blastoderm over 

 the egg (Piate 4 figs. 5 and 6). 



Soon after the blastoderm is complete an invagination begins at the equator of the 

 egg (fig. 7) and soon a complete little bag is formed (fig. 8); this is the gastmla stage. 

 That a very perfect gastrulation should occur in this manner is of high interest, because 

 neither in the Nauplius nor in any later stage of the life history is there any trace of gut 

 or endoderm to be observed. Soon after the gastrulation is complete the cells which form 

 the endoderm bag break apart and lie in the middle of the yolk (fig. 9). Here they remain 

 for a short time, but they rapidly begin to degenerate, and though they may be observed 

 lying in the yolk after the rudiments of the limbs have begun to be formed (Piate 4 fig. 11), 

 after this stage no sign of them can be detected. . The perfect gastrulation, which also occurs 

 in the embryogeuy of Sacculina, is therefore merely an ancestral rudiment and has no rela- 

 tion, that we can see, to the present requirements of the animai, which does not develope 

 a functional gut at any period of its existence. 



The rudiments of the future Nauplius are first laid down as two lateral proliferations 

 of the blastoderm which soon become segmented in the manner shown in fig. 10. The first 

 antennal segment remains continuous with the acron after the second antennal and mandi- 

 bular segments have been completely divided off. The blastoderm in the dorsal and 

 ventral regions remains one celi thick at present. A horizontal section at this stage (fig. 11) 

 shows that the segmentai proliferations are solid and many cell-layers thick. In the next stage 

 (Piate 4 fig. 12), another continuous unsegmented proliferation of the blastoderm has occurred 

 in the ventral middle line (VP); this ventral piate ends abruptly, at a little below the egg's 

 equator, in a transverse furrow, which gives the young Nauplius its characteristic shape. At 

 the posterior pole of the egg certain large cells with conspicuous nucleoli have begun to 

 differentiate (Th), which are the rudiments of the thorax of the future Cypris. The blasto- 

 derm on the dorsal surface is stili one celi thick. The first antenna is from the first uni- 

 ramous, the 2 nd antenna and mandible are already bifurcated. The rudiments of the frontal 

 horns are marked out by some conspicuous cells (Fh). 



Piate 4 fig. 13 is a ventral view of a slightly later stage in which ali these structures 

 can again be observed. The horizontal section drawn in fig. 14 shows some interesting internai 

 structures: the section passing through the ventral piate and the appendages. At the base 

 of the second antenna are to be observed two little vesicles (Ves) which I take to represent 

 the antennal gland characteristic of entomostracan Nauplii. They appear to be entirely closed 

 and I cannot determine if they are developed from the ectoderm or mesoderm. Lying just 

 posteriorly to them are a pair of skeletogenous elements (sk) which serve as attachments for 

 the muscles which move the limbs. The large cells of the thoracic rudiment (Th) are also 

 shown. In the transverse section (fig. 15) at this stage we again see the vesicle of one side and 

 the paired skeletogenous elements from which muscles (muse) pass dorsally. The transverse 



